The telephone industry has always attempted to maintain high standards in its provisions of services to the public. Part of this effort in the past has involved service observation of non-conversational portions of arbitrarily selected telephone calls. The observed and evaluated operations include operator responses and procedures, equipment delays, disposition of calls, etc. At one time service observing was predominantly manual requiring the use of many personnel. In one effort to reduce the manual effort required for this purpose, and also to minimize as much as possible the extent of access of a service observing operator to a call, the Bell System some time ago introduced the Service Evaluation System (SES) No. 1. This system automated many of the operations previously performed by service observing personnel. At the present time, efforts to further automate these operations are being made. In particular, one aspect of such efforts is in the automatic detection of calls completing to announcement machines and the identification of the types of announcements encountered by such calls. Such announcements pertain, for example, to changed number, interrupt, vacant code, call blocked announcements, and the like.
To provide the capability for a machine to differentiate between the speech of a recorded announcement and that of a normally answered call, special information tones, called SIT tones, are magnetically recorded on analog recording drums in telephone offices in conjunction with the announcements. SIT tones used by the Bell System consists of a sequence of tone bursts (pulses) in which each pulse is encoded in both frequency and duration. The tones are broadcast immediately before the announcement with which they are associated. The presence of the SIT tones differentiates a recorded announcement from a called party answer. The frequency and duration encoding of the SIT tones uniquely identify the particular type of announcement in question.
The actual frequencies of the SIT tones that are generated and transmitted when a recorded announcement is made by a rotating device in a particular telephone office is dependent on the rotational speed of the device. Measurements that have been taken of mechanical drum recorders in use by telephone companies show that drum rotation speeds of such recorders vary from specification by as much as .+-.5 percent. This speed variation is perfectly satisfactory to reproduce high quality, intelligible speech signals. It is inadequate, however, to reproduce encoded tone signals that must be selectively and reliably decoded. For example, a nominal 1800 hertz per second tone on such a device might actually be reproduced as a signal anywhere in the range of 1710 to 1890 hertz per second. The problem of automatically classifying such a signal is worsened by the fact that the practical choice of encoding frequencies is limited in the intended telephone network environment in that they must not conflict with other conventional tone signals used for signaling in the telephone network.